Metal foils of, for example, metallic lithium have been used in both primary and secondary electrochemical cells. In a lithium-sulphur cell, for instance, lithium metal foil may be used as the anode of the cell.
In order to improve the specific energy of, for example, a lithium-sulphur cell, it is desirable to reduce its overall mass. In theory, this may be achieved by reducing the thickness of the electrodes since the electrochemical reactions during charge and discharge only occur at the surface of the electrode. Thin lithium foil, however, is very soft and is easily bent and/or torn. When such foils are cut for use, the foils also have a tendency to stick to the blades used in the cutting procedure. As a result, thin lithium foils are extremely difficult to handle and produce. In fact, the typical thickness of commercially available lithium foil is 100 μm or greater.
It is known from U.S. Pat. No. 3,721,113 to provide a process for rolling thin continuous lithium strips in thicknesses less than 400 μm by cold rolling lithium metal while it is compressed between smooth surfaces of a solid polymeric composition. It is stated that thicknesses down to about 40 μm are achievable but this is not exemplified. The solid polymeric composition may be in the form of the surfaces of a pair of rollers, or may be a pair of polymer sheets that sandwich a strip of lithium. It is important to appreciate, however, that the polymer sheets are peeled away from the lithium foil after cold rolling, and are not intended to act as a support in order to improve subsequent handling. Accordingly, although the metal lithium foils disclosed in this document may be thin, they are difficult to work with once produced.
US 2009/0246626 describes a lithium ion cell in which lithium metal foil is used as a source of lithium ions. In particular, US 2009/0246626 describes a lithium ion cell comprising positive electrodes and negative electrodes formed from carbon. To initialize the cell, the negative electrodes are first doped with lithium ions from the lithium metal foil. Specifically, the lithium metal foil is placed in electrical contact with the negative electrodes in the presence of an electrolyte. After a period of time, the lithium metal foil completely dissolves to form lithium ions which intercalate or dope the negative electrodes. Once dissolved, therefore, the metal foil plays no part in the cell's charge and discharge chemistry.
US 2009/0246626 recognises the difficulties inherent in handling thin lithium foils, and proposes a lithium metal foil provided on one side or on both sides with a support member formed of paper or resin non-woven fabric that is adhesive or pressure bonded to the lithium foil. The reference, however, is not concerned with reducing the thickness of a lithium foil. Instead, the reference states that the thickness is not limiting and is determined by the amount of lithium ions doped into the cell and the area of the lithium metal foil. Since a single sheet of lithium foil may be used to intercalate or dope a series of electrodes in a stack, a thickness of 50 to 300 microns is said to be preferred. Although the thickness of the support member is said to be preferably 20 to 100 microns, the reference does not disclose the thicknesses of any bonded structures. In fact, although pressure bonding is mentioned, this need not result in firm fixation but may merely be sufficient to ensure that the foil and support member are not misaligned during subsequent cutting and handling. There is no suggestion that any reduction in thickness can be achieved by using a composite structure. Indeed, the addition of a support would be expected to increase the thickness of the resulting composite.
EP 1865520 describes a lithium electrode formed by contact bonding a sheet of lithium metal to stainless steel net. The prior art document mentions the possibility of applying a sheet of lithium metal to either side of the current collector. However, EP 1865520 does not describe the step of rolling or otherwise pressing and stretching the composite to substantially reduce its overall thickness. Indeed, the Example describes an electrode having a thickness of 148 μm that is formed by contact bonding a single sheet of lithium metal to a stainless steel net. It should also be noted that the lithium electrode described in EP 1865520 is not employed as the working anode of the electrochemical cell but simply as a source of lithium ions for an anode formed, for example, of graphite for reversibly intercalating lithium ions. Moreover, since the stainless steel net is used as a current collector, it is necessarily conducting. On exposure to electrolyte, therefore, it may become a centre for dendrite formation. This is generally undesirable.
US 2004/0072066 describes a lithium electrode that is formed by depositing a lithium metal layer onto a porous polymer film using, for example, vapour deposition. The porous polymer film is present on and integrated with the electrolyte-facing surface of the lithium electrode. A protective coating layer having lithium ion conductivity but that is impermeable to electrolyte may be provided between the porous polymer film and the lithium metal layer. The aim of US 2004/0072066 is to provide layers over the surface of the lithium metal electrode.
In view of the foregoing, it is among the objects of the present invention to improve the specific energy of an electrochemical cell.
It is also among the objects of the present invention to reduce the thickness of a metal foil electrode.